Take a look at the next desktop PC or laptop you come across. Odds are good it won't be running an open-source operating system. Microsoft's closed-source Windows has by far the highest share of the PC client operating system market, followed in a distant second by Apple's Mac OS X. Linux and other wholly open source operating systems have only a tiny market share.

It's not hard to see why. Despite the advances made by distributions such as Ubuntu, desktop Linux is still miles behind Windows and OS X in terms of the look, the feel and the slickness that most office workers have come to expect. The vast majority of companies simply aren't prepared to make office workers use an open source OS — and most office workers aren't prepared to use them, either.

Even if you want to stick with a closed source operating system (or, the case of OS X, partially closed source), your business can still take advantage of a vast amount of open source software. The most attractive benefit of doing so: It's generally available to download and run for nothing. While support usually isn't available for such free software, it's frequently offered at an additional cost by the author or a third party. It may be included in a low-cost commercially licensed version as well.

Is it possible, then, to run a business entirely on software that can be downloaded for free? There certainly are many options that make it possible — and many more that aren't included in this guide.

Paul Rubens is a technology journalist based in England. Contact him at paul@rubens.org. Follow everything from CIO.com on Twitter @CIOonline, Facebook, Google + and LinkedIn.

Open Source Office Productivity Suites

Very few companies using Microsoft Office actually require support from Microsoft, so using an open source alternative can make good financial sense. Open-source suites are compatible with Microsoft Office file formats such as .doc and .xls. Though their feature sets aren't quite as comprehensive as Office, that's unlikely to matter — most people only use a fraction of the available features available anyway.

Open Source Finance and Accounting Applications

These three financial packages provide open sources alternatives to commercial packages such as Quicken, Sage or QuickBooks.

GnuCash (Windows, OS X and Linux) helps very small businesses apply professional accounting principles and track bank accounts, income and expenses to help ensure balanced books and accurate financial reports.

TurboCASH (Windows), aimed at the small to medium-sized business market, provides a general ledger and offers core functionality such as posting transactions into accounts and producing financial reports. The product supports 80 tax regimes, including the United States and the United Kingdom.

Finally, xTuple offers both a free and commercial supported version of Postbooks (Windows, OS X, Linux, BSD) to medium and larger companies. It includes accounting (general ledger, accounts receivable and payable, bank reconciliation and financial reporting) as well as sales, CRM, inventory and distribution, and other business management functions.

OpenERP (Windows and Linux) provides a full suite of applications, including sales management, accounting and finance, recruitment, purchase management and manufacturing resource management. A paid version — which includes support, automated updates and an unlimited bug fix guarantee — is available for € 35 (about $50) per user per month.

Open Source CRM Software Options

Commercial customer relationship management (CRM) software is available from niche vendors, software as a service providers such as Salesforce.com and large software vendors such as Oracle. Here are three open source alternatives:

The SugarCRM Community Edition (Windows, OS X, Linux and UNIX) is the free, unsupported version of a comprehensive CRM product that includes sales force automation, marketing campaigns and customer support.

OpenCRX (Windows, OS X and Linux) is designed for organizations that need multifunctional, enterprise-wide coordination of sales generation, sales fulfillment, marketing and service activities for customers, partners, suppliers and intermediaries.

The multi-platform Fat Free CRM is a Ruby on Rails-based CRM platform that features group collaboration, campaign and lead management, contact lists and opportunity tracking out of the box.

Open Source Communications and Telephony Software

Private branch exchange (PBX) hardware used to be very expensive, but software-based PBX releases bring the price of PBXs down. The open source PBX market is dominated by Asterisk software.

AsteriskNOW (Linux) is an easy-to-install IP PBX that comes with the FreePBX administrative GUI. The Asterisk project is sponsored by Digium, which also offers a commercial version of the PBX.

Elastix (Linux) is based on Asterisk telephony software, but it also integrates fax, instant messaging and email functionality from other open source projects. Features include voicemail, fax-to-email, support for softphones, virtual conference rooms and call recording.

Open Source Content Management Systems

Content management systems power websites — and many of the world's largest sites use an cross-platform, open source CMS such as the following:

Big-name sites such as The Economist, Examiner.com and The White House use Drupal. Features include user account registration and maintenance, menu management, RSS feeds, page layout customization and system administration.

WordPress started as a blogging system but has evolved into a full-blown CMS with thousands of plugins, widgets and themes. It's the most popular blogging platform on the Web and powers about 20 percent of the top 10 million sites on the Internet.

Second only to WordPress in implementations, Joomla is used by organizations including Harvard University and Citibank, and is. Thousands of extensions, both free and commercial , are available from the Joomla! Extension Directory.

Java-based OpenCMS offers a browser-based work environment (including a WYSIWYG editor) as well as asset, user and workflow management.

Open Source Ecommerce Tools

Ecommerce software packages, such as the three cross-platform products listed here, include everything from product presentation to shopping carts, checkout and payment processing.

OpenCart is a turnkey shopping cart for small and medium-sized retailers. It includes order management and multiple payment gateways, provides the capability for customers to leave product reviews and ratings, and offers lifetime free community support. (Commercial support is available through an international partner network.)

PrestaShop is a popular and easy-to-use platform that offers product displays, translations, marketing, localization and taxes, and product export to eBay. Support is available as a paid service. Training is offered in France.

An older ecommerce platform that branched from osCommerce, Zen Cart is aimed at developers and advances users. Features include discount coupons, gift vouchers and multiple payment options.